Device for achieving aim-off for a firearm

ABSTRACT

A gun is described in which a gun barrel and a gun sight are mounted for movement together in deploying the gun in azimuth and in elevation under the action of servo drive means. A tracking unit is provided to control the servo drive means, which may comprise separate servo motors for rotating an upper mounting of the gun in azimuth and for rotating the gun barrel in elevation relative to the upper mounting. The tracking unit has two separate phases of operation. In its first phase of operation, the tracking unit produces controlling signals which represent the behaviour of a target as sensed by a gun layer controlling an aiming device and a rangefinder to produce aiming and range signals which form inputs to the tracking unit. In its second phase of operation, the tracking unit produces controlling signals which represent a predicted behaviour of the target by maintaining constant a function of the controlling signals which is determined by the particular nature the controlling signals take and the assumed behaviour of the target. When the controlling signals represent the Cartesian coordinates of the position vector of the target, and it is assumed that the target is moving with constant velocity along a straight line at the time of firing the gun, the time derivative of the controlling signals is maintained constant. Auxilliary circuits are described by which aim-off corrections may be made on the basis of the estimated time of flight of a projectile to the target depending on known ballistic properties of the projectile and on ambient atmospheric conditions.

States Patent [1 1 Salomonsson 1 Oct. 23, 1973 DEVICE FOR ACHIEVING AIM-OFF FOR A FIREARM [75] Inventor: Hans Manne Alvar Salomonsson,

Karlskoga, Sweden [73] Assignee: Aktiebolaget Bofors, Bofors, Sweden [22] Filed: Feb. 8, 1972 [21] Appl. No.: 224,470

[30] Foreign Application Priority Data Feb. 26, 1971 Sweden 2440/71 [52] U.S. Cl 89/41 B, 89/41 E, 89/41 L, 235/61.5 E [51] Int. Cl. F41g 3/06 [58] Field of Search 33/238, 239;

89/41R,41E, 41 AA, 41 M, 41 L, 41 B, 41 SW; 235/6l.5 R, 61.5 E, 61.5 DF, 61.5 S;

Primary Examiner-Stephen C. Bentley Att0rneyWilliam D. Hall et a1.

[57] ABSTRACT A gun is described in which a gun barrel and a gun sight are mounted for movement together in deploying the gun in azimuth and in elevation under the action of servo drive means. A tracking unit is provided to control the servo drive means, which may comprise separate servo motors for rotating an upper mounting of the gun in azimuth and for rotating the gun barrel in elevation relative to the upper mounting. The tracking unit has two separate phases of operation. In its first phase of operation, the tracking unit produces controlling signals which represent the behaviour of a target as sensed by a gun layer controlling an aiming device and a rangefinder to produce aiming and range signals which form inputs to the tracking unit. In its second phase of operation, the tracking unit produces controlling signals which represent a predicted behaviour of the target by maintaining constant a function of the controlling signals which is determined by the particular nature the controlling signals take and the assumed behaviour of the target. When the controlling signals represent the Cartesian coordinates of the position vector of the target, and it is assumed that the target is moving with constant velocity along a straight line at the time of firing the gun, the time derivative of the controlling signals is maintained constant. Auxilliary circuits are described by which aim-off corrections may be made on the basis of the estimated time of flight of a projectile to the target depending on known ballistic properties of the projectile and on ambient atmospheric conditions.

13 Claims, 6 Drawing Figures SVr PAIENTEDUCI 2 3 I973 SHEET 10F 3 \A q am PATENTEI] OCT 23 I973 SHEET 2 BF 3 uou 2 2 m no n ow n 2 3 .NN U M QQ w k 6& xx Ex 2N 2: m *x 2 om m2 PATENTED OCT 23 I973 SHEET 3 BF 3 DEVICE FOR ACHIEVING AIM-OFF FOR A FIREARM BACKGROUND OF THE INVENTION This invention relates to guns adapted to be deployed in azimuth alone to shoot at a target and to guns which are adapted to be deployed both in azimuth and elevation to shoot at a target.

Guns such as anti-aircraft guns have been previously proposed in which inter alia a gun sight and calculating means for evaluating target data and/or calculating aim-off have been employed. As a rule, the gun sight and data processing and calculating means have been mounted separately. This has involved disadvantages in the form of comparatively slow deployment, limited accuracy in view of parallax corrections which have been required, and less reliability because of the requirement for cables interconnecting the gun itself and the gun sight and data processing and calculating means.

Attempts to mount the gun sight on the gun mounting have not been altogether successful. Optical vision tends to become obscured by smoke produced during firing of the gun which disturbs the coupling of the gun and the gun sight in setting the aim-off, and which makes the gun layers task difficult. An example of one such prior proposal which has not been altogether successful is the so-called dependent line-of-sight control.

A primary object of the present invention is to provide guns in which the gun sight is mounted for movement with the gun barrel to allow more rapid deployment, better accuracy and reliability, and without being unduly disturbed by smoke produced at firing.

It is a further object of this invention to provide a gun which makes the gun layers task easier.

BRIEF DESCRIPTION OF THE INVENTION According to this invention, there is provided a gun adapted to be deployed for shooting at a target, the gun comprising:

a gun barrel and a gun sight;

common servo drive means for moving the gun barrel and the gun sight together;

aiming and rangefinding means controllable by a gunlayer for producing aiming and range signals; and

a tracking unit adapted to produce signals for controlling the servo drive means and having two phases of operation, the tracking unit being arranged in its first phase of operation to produce said signals in dependence on the aiming and range signals such that the controlling signals represent the behaviour of the target as sensed by the gunlayer, and being arranged in its second phase of operation to maintain a predetermined function of said controlling signals constant such that the controlling signals represent a predicted behaviour of the target.

Preferably the tracking unit further includes means for producing correction signals to achieve aim-off, and comprising means for estimating the time of flight of a projectile to the target from data signals present in the tracking unit and from additional data input signals dependent on known properties of the projectile employed and on weather conditions, means for generating aim-off correction signals in response to the estimated time of flight, and means for modifying the controlling signals in accordance with the respective aimoff correction signals.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows schematically elements of an embodiment of anti-aircraft gun system constructed in accordance with the present invention;

FIG. 2 shows in somewhat greater detail part apparatus shown in FIG. 1;

FIGS. 3a and 3b shows schematically two embodiments of coordinate converters useful in the constructions illustrated in FIGS. 1 and 2;

FIG. 4 shows schematically a time-of-flight calculator; and

FIG. 5 shows schematically, but in more detail than in FIG. 4, an embodiment of time-of-flight calculator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawings show schematically elements of an embodiment of anti-aircraft gun system constructed in accordance with the present invention. The gun may comprise an upper mounting arranged to traverse in azimuth on a lower mounting which may be placed on the ground, and a gun barrel and associated firing mechanism arranged to be deployed in elevation on the upper mounting. Equipment for evaluating target data is mounted either directly to the gun barrel, or separately on the upper mounting and supported via a mechanical linkage to move with the gun barrel. This equipment includes an optical sight, with the aid of which an operator can observe the deviation of the line of fire (or the axis of the gun barrel bore) from the line of sight to a target; and an optical rangefinder, from which the range to the target is obtained automatically when this is in the line of sight. The upper mounting is traversed in azimuth and the gun is deployed in elevation by servo drive means which comprise servo motors with associated servo amplifiers. Computing elements are connected to the shafts of the servo drive means via mechanical transmissions; in the preferred embodiment these computing elements consist of resolvers which may be of a kind well known per se.'A position is provided for a gunlayer on the upper mounting. Aiming means coupled with the gun sight, and under the control of the gunlayer are provided on the upper mounting. The aiming means is arranged to provide aiming signals in two coordinates for correcting the direction of the gun so that the axis of the gun barrel bore can be made to coincide with the line of sight. The tracking unit is arranged to perform the necessary data of the processing of the raw aiming and range signals produced by the aiming and rangefinding means to convert these into signals for controlling the servo drive means. As is explained, the tracking unit has two distinct phases of operation, in the second of which assumptions 7 are made as to the behaviour of the target, such as that it moves with a constant speed along a straight course. The tracking unit is also arranged to provide an appropriate aim-off dependent on given ballistic properties of the projectile, atmospheric conditions, etc. During the stage when the target data is measured, the tracking is also facilitated in that the tracking unit also during this stage calculates aiming coordinates based upon target indication, rangefinding and aiming-device corrections.

Referring to FIG. 1, there are shown schematically first and second servo drive means which may be of designs which are known per se. The drive means respectively comprise a first servo motor l, by which the gun barrel 100 in its upper mounting 101 is rotatable in elevation about the axis of a first shaft hv, and a second servo motor 2, by which said upper mounting 101 is rotatable in azimuth about the axis of a second shaft sv. Respective resolvers 3 and 4 are provided at the shafts in a manner known per se for sensing their angular positions. A gun sight 102 and a rangefinder 8, which may be of a design known per se such as a laser-type rangefinder are fixed to the gun barrel 100.

The tracking unit generally indicated 11 produces controlling signals x,,,, y,,, and 2,, (referred to a Cartesian coordinate system). These are transmitted to the servo drive means where they cause control signals (error signals)e,,,, and e for the servo motors I and 2 to be produced. These control signals are transmitted to the servo motors via respective servo amplifiers 5 and 6. The servo drive means can be considered in part as a coordinate converter, in which the controlling signals x,,,, y,,, and z, referred to a Cartesian coordinate system are converted into signals in a polar coordinate system. In addition to the said error signals, a signal r, calculated in the servo drive means and representing the slant range to the target is also obtained.

The input signals to the tracking unit comprise aiming correction signals, referred to a polar coordinate system centred at the gun (indicated hv, and sv, in the drawings) produced by means of an aiming device 7 under the control of the gun layer (as explained above) and a third range signal is achieved with the aid of the slant range signal r, generated in the servo drive means and a range signal r for the actual range in the polar coordinate system, which signal r,, is obtained from the rangefmder 8.

FIG. I also shows symbolically the addition to the controlling signals x,,,, y,, and z, of additional aim-off signals x,, y, and z, via the switch means 9a, 9b and 9c and adders 10a, 10b and 10c, which are shown symbolically in the figure. The resultant signals are indicated x,,, y, and 2;, in FIG. 1. FIG. 2 shows the construction of the tracking unit in more detail but still symbolically. The signals r, and r are added together in an adder 12, from which a resultant range correction signal r, is obtained. The aiming and range signals hv,, sv and r are transmitted to a coordinate converter 13, which converts these signals, which are in polar coordinate form, to corresponding signals x,, y, and z in a Cartesian coordinate system. The converted signals provide the inputs to a signal-processing unit 114, which comprises for each coordinate a respective integrator a, 15b, llSc and a respective adder 16a, Mb, 166, the arrangement being such that each signal is added to the integral of the signal itself. In this way, signals v,, v and v, are formed which represent the velocity components of the target in the Cartesian coordinate system, or in other words, represent the time derivatives of the aiming coordinates of the target.

When a target appears which is characterized by a constant speed along a straight course, the signals v v,, and v, become constant, and the signals x,,,, y,,, and z,,,, for controlling the servo drive means which signals correspond to the aiming coordinates of the target, can be generated with the aid of further integrators 17a, 17b, and I70, the input values of which represent said time derivatives. When the gun layer, with the aid of the aiming means, has established accurate tracking of the target, these time derivatives, which are thus constant, can be locked with the aid of respective retaining circuits 18a, 18b and 18c, which are provided with switch means (not shown), and which may be of a design known per se. The sight setting will then automatically follow the line of aim to the target provided that no additional corrections for aim-off, tangent elevation or other factors have been introduced. At the switch over to this second phase of operation of the tracking unit in which the time derivatives are maintained constant, the aiming and the rangefinding corrections are effectively disengaged. Thereafter, it is not necessary to be able to observe the target via the sight.

Aim-off correction signals x,, y, and z, are obtained in additional circuits which contain multipliers 19a, 19b and I90, which are connected in parallel with the respective integrators l7a, 17b and 170. The multipliers are arranged to be connected by switch means 20a, 20b and 20c for addition of the aim-off signals to the controlling signals x,,,, y, and z,,, via adders 21a, 21b and 210.

The signals x,, y, and z, are arranged to be added to the controlling signals x,,,, y,,, and z,,, at the same instant as the tracking unit switches over to its second phase of operation (i.e. locking on to automatic tracking), or immediately thereafter. The signals x y and z thus obtained are used to control the servo motors during the second phase of operation so that the gun is instantaneously aimed at the calculated aiming point arrived at on a prediction of the behaviour of the target. The signal r, obtained in the servo drive means, which signal represents the slant range to the aiming point, is transmitted to the tracking unit, and corrections of further values are based upon this signal through a trial and error method. As soon as the gun has slewed on to the target, which in normal cases can take 0.5 1 second, firing takes place.

The switch means for the retaining circuits 18a, 18b, and 18c, and the switch means 20a, 20b and 200 are preferably coupled to the firing mechanism of the gun in such a way that the switch means 20a, 20b and 200 are connected in simultaneously with or immediately after the connection of the retaining circuits, and that the time interval between the connection of the retaining circuits and the actual firing of the gun is comparatively short.

As will be seen from FIG. 2, the signal-processing unit 14, includes switch means 22a, 22b and 22c, for connecting and disconnecting the respective integrators 15a, 15b and 150. These switch means are arranged during disconnection to give the integrators appropriate starting values. This facilitates the work for the gun layer considerably during the stage when slewing on to the target.

FIG. 3a shows schematically one embodiment of coordinate converter 13. The converter is constructed in a manner known per se and comprises two resolvers 24 and 25 one of which, 24, senses the angular position of the shaft hv and the other, 25, the angular position of the shaft sv. The embodiment of FIG. 3a, and the description with respect of FIG. 2 both involved the use of a coordinate converter for a Cartesian coordinate system involving three coordinates. This is appropriate for guns intended to follow a target in space. In cases when a gun is specifically intended for use against targets at sea or on ground, it may be sufficient to employ a Cartesian coordinate system involving only two coordinates, in which case the coordinate converter can be simplified as shown in FIG. 3b. The manner in which such a simplified converter would be incorporated in a weapon system referring only to two Cartesian coordinates, and how the other parts of this weapon system would be arranged should be obvious having regard to the above description of a three coordinate system, and will therefore not be described herein.

In the additional circuits aim-off signals 1: y, and z, are formed with the aid of the multipliers 19a, 19b, and WC, respectively, according to the equations in which 2,, the estimated time of flight of the projectile.

FIG. 4 shows a time-of-flight calculator 26, which calculates the time of flight with the aid of the slant range signal r, generated in the servo drive means, and transmits a time-of-flight signal 2, which represents this. The time-of-flight calculator has setting devices in the form of knobs for setting correction values corresponding to varying muzzle velocities (A V and varying atmospherical conditions (A 8).

FIG. shows a more detailed example of a time-offlight calculator, comprising an integrator 27, on the input of which three different voltages are added. Setting devices 29 and 30 consisting of e.g. change-over switches or potentiometers are employed to set the values l/V which is dependent upon A V,,, and c, which is dependent upon A 6. The output of device 2? provides one of the inputs to integrator 27; a second input comprises a signal fed back from the output of integrator 27 and a multiplier 28 receiving its two inputs from the outputs of setting devices 29 and 30, as shown in FIG. 5, provides the third input voltage to integrator 2'7. The calculator functions as a feed-back amplifier with high amplification, and therefore the output voltage t, assumes a value such that the algebraic sum of the input voltages on the integrator becomes zero. The relation between t, and r, is then Experiments show that the relation between r, and i according to a range table can be reproduced with a high degree of accuracy from this equation provided the values V and c are appropriately chosen.

Referring now particularly to FIG. 1, it will be seen that the resolvers 3 and 4 shown carry out coordinate conversions according to the following relations:

The coordinate commands to the servo motors are obtained according to the following equations:

During the first phase of operation of the tracking unit:

in which x y,,, and z are starting values obtained l I from an indication system.

During the second phase of operation of the tracking unit:

in which x y and z are the target coordinates at the instant of locking on to automatic tracking.

The invention is not limited to the embodiment specifically described above by way of example, but can be subject to modifications within the scope of the appended claims. Thus, for instance, it is possible to carry out conversions from the signal values-(hv sv r,, and n) forming the inputs to the tracking unit 11 and the output controlling values (x y and z to the servo drive means in different ways. Moreover, the additional circuits providing aim-off are not confined to the specific embodiment shown in the drawings. The tracking unit can be made to transmit signals that represent acceleration components of the target rather than velocity components. The tracking unit can also be arranged in its second phase of operation to maintain constant several different signals, for instance, signals corresponding to velocity can be generated in the tracking unit and maintained constant at the same time as signals representing acceleration.

What is claimed is:

l. A deployable gun for shooting at a target, the gun comprising:

a gun barrel having a gun sight connected thereto with the line of aim of said sight always being in fixed alignment with the line of said barrel;

common servo drive means for moving the gun barrel and the gun sight together;

aiming and rangefinding means controllable by a gunlayer for producing aiming and range signals; and

a tracking unit operative to produce signals for controlling the servo drive means and having two phases of operation, the tracking unit including means operative in its first phase of operation to produce said signals in dependence on the aiming and range signals such that the controlling signals represent the behaviour of the target as sensed by the gunlayer, and said tracking means including means operative in its second phase of operation to maintain at least the target velocity components of said controlling signals constant whereby the controlling signals represent a predicted behaviour of the target.

2. A gun according to Claim 1, wherein the gun sight and the gun barrel are affixed to one another, said servo drive means comprising a first servo motor for rotating the gun barrel and the gun sight together in elevation, and a second servo motor for rotating the gun barrel and the gun sight together in azimuth.

3. A gun according to claim 1, wherein the gun barrel and the gun sight are connected to one another via a mechanical linkage, said servo drive means comprising a first servo motor for rotating the gun barrel and the gun sight together in elevation, and a second servo motor for rotating the gun barrel and the gun sight together in azimuth.

4. A gun according to claim 1, wherein the controlling signals produced by the tracking unit represent components of the velocity of the target in a Cartesian coordinate system, and wherein the tracking unit is arranged in its second phase of operation to maintain the time derivatives of the controlling signals constant.

5. A gun according to Claim 1, wherein said servo drive means comprises a first servo motor for rotating the gun barrel in elevation and a second servo motor for rotating the gun barrel in azimuth; said servo motors being operative to generate elevation and azimuth error signals to servo control movement of the gun barrel in operation; said servo motors being further operative to produce a slant range signal referred to a polar coordinate system centered at the gun, and means for transmitting the slant range signal to the tracking unit in addition to the range signal, whereby the controlling signals produced by the tracking unit are at least partly dependent on the slant range.

6. A gun according to claim ll, wherein the controlling signals produced by the tracking unit represent the components of the position of the target in a Cartesian coordinate system; and wherein the tracking unit comprises respective integrators for the respective Cartesian coordinates adapted to produce at their respective outputs the said controlling signals; and the tracking unit further comprises means for determining the components of the velocity of the target in a Cartesian coordinate system from the aiming and range signals, and means for transmitting said velocity component signals to the respective inputs of the respective integrators.

'7. A gun according to Claim 1, wherein the aiming and rangefinding means are arranged to produce aiming and range signals which are in the form of correction signals referred to a polar coordinate system centered at the gun; and wherein the tracking unit comprises means for converting the aiming and range correction signals in polar form to correction signals referred to a Cartesian coordinate system, and a signal processing unit comprising for each Cartesian coordinate an integrator and an adder arranged together to produce an output signal representing the sum of the Cartesian correction signal and its integral.

8. A gun according to claim 7, wherein the tracking unit is arranged in its second phase of operation to maintain the values of the respective output signals constant.

9. A gun according to claim 1, wherein the tracking unit further includes means for producing correction signals to achieve aim-off and comprising means for estimating the time of flight of a projectile to the target from data signals present in the tracking unit and from additional data input signals dependent on known properties of the projectile employed and on weather conditions, means for generating aim-off correction signals in response to the estimated time of flight, and means modifying the controlling signals in accordance with respective aim-off correction signals.

10. A gun according to claim 9, wherein means are provided for automatically adding the aim-off correction signals to the controlling signals during the second phase of operation of the tracking unit.

11. A gun according to claim 9, wherein means are provided for automatically adding the aim-off correction signals to the controlling signals at the change over from the first phase of operation to the second phase of operation.

12. A gun according to Claim 6, wherein the tracking unit further comprises means of achieving aim-off, and comprising for each of the controlling signals generated by the tracking unit a multiplier connected in parallel with said integrator; calculating means for providing signals corresponding to the estimated time-of-flight of a round to the multipliers, the calculating means being arranged to produce time of flight signals in response to a first input representing the slant range and second and third signals respectively representing correction values for varying muzzle velocities of rounds of ammunition of different kinds and correction values relating to variations in atmospheric conditions, means being provided for manually setting said values in the calculating means; and means for adding the aim-off correction signals produced by the respective multipliers to the respective controlling signals, at a time not preceding the commencement of the second phase of operation of the tracking unit.

13. A gun according to claim 12, wherein the calculating means comprises an integrator, means for generating a first ammunition-dependent signal which is an inverse function of the muzzle velocity of the round and which is a function of the slant range, means for generating a second signal which is a function of the at mospheric conditions, a multiplier for multiplying said first and second signals together, and an integrator whose input signals comprise said first signal and the result of multiplying said first and second signals and which integrator is provided with a negative feedback loop, the time-of-flight calculated signal consisting of the output of said integrator. 

1. A deployable gun for shooting at a target, the gun comprising: a gun barrel having a gun sight connected thereto with the line of aim of said sight always being in fixed alignment with the line of said barrel; common servo drive means for moving the gun barrel and the gun sight together; aiming and rangefinding means controllable by a gunlayer for producing aiming and range signals; and a tracking unit operative to produce signals for controlling the servo drive means and having two phases of operation, the tracking unit including means operative in its first phase of operation to produce said signals in dependence on the aiming and range signals such that the controlling signals represent the behaviour of the target as sensed by the gunlayer, and said tracking means including means operative in its second phase of operation to maintain at least the target velocity components of said controlling signals constant whereby the controlling signals represent a predicted behaviour of the target.
 2. A gun according to Claim 1, wherein the gun sight and the gun barrel are affixed to one another, said servo drive means comprising a first servo motor for rotating the gun barrel and the gun sight together in elevation, and a second servo motor for rotating the gun barrel and the gun sight together in azimuth.
 3. A gun according to claim 1, wherein the gun barrel and the gun sight are connected to one another via a mechanical linkage, said servo drive means comprising a first servo motor for rotating the gun barrel and the gun sight together in elevation, and a second servo motor for rotating the gun barrel and the gun sight together in azimuth.
 4. A gun according to claim 1, wherein the controlling signals produced by the tracking unit represent components of the velocity of the target in a Cartesian coordinate system, and wherein the tracking unit is arranged in its second phase of operation to maintain the time derivatives of the controlling signals constant.
 5. A gun according to Claim 1, wherein said servo drive means comprises a first servo motor for rotating the gun barrel in elevation and a second servo motor for rotating the gun barrel in azimuth; said servo motors being operative to generate elevation and azimuth error signals to servo control movement of the gun barrel in operation; said servo motors being further operative to produce a slant range signal referred to a polar coordinate system centered at the gun, and means For transmitting the slant range signal to the tracking unit in addition to the range signal, whereby the controlling signals produced by the tracking unit are at least partly dependent on the slant range.
 6. A gun according to claim 1, wherein the controlling signals produced by the tracking unit represent the components of the position of the target in a Cartesian coordinate system; and wherein the tracking unit comprises respective integrators for the respective Cartesian coordinates adapted to produce at their respective outputs the said controlling signals; and the tracking unit further comprises means for determining the components of the velocity of the target in a Cartesian coordinate system from the aiming and range signals, and means for transmitting said velocity component signals to the respective inputs of the respective integrators.
 7. A gun according to Claim 1, wherein the aiming and rangefinding means are arranged to produce aiming and range signals which are in the form of correction signals referred to a polar coordinate system centered at the gun; and wherein the tracking unit comprises means for converting the aiming and range correction signals in polar form to correction signals referred to a Cartesian coordinate system, and a signal processing unit comprising for each Cartesian coordinate an integrator and an adder arranged together to produce an output signal representing the sum of the Cartesian correction signal and its integral.
 8. A gun according to claim 7, wherein the tracking unit is arranged in its second phase of operation to maintain the values of the respective output signals constant.
 9. A gun according to claim 1, wherein the tracking unit further includes means for producing correction signals to achieve aim-off and comprising means for estimating the time of flight of a projectile to the target from data signals present in the tracking unit and from additional data input signals dependent on known properties of the projectile employed and on weather conditions, means for generating aim-off correction signals in response to the estimated time of flight, and means modifying the controlling signals in accordance with respective aim-off correction signals.
 10. A gun according to claim 9, wherein means are provided for automatically adding the aim-off correction signals to the controlling signals during the second phase of operation of the tracking unit.
 11. A gun according to claim 9, wherein means are provided for automatically adding the aim-off correction signals to the controlling signals at the change over from the first phase of operation to the second phase of operation.
 12. A gun according to Claim 6, wherein the tracking unit further comprises means of achieving aim-off, and comprising for each of the controlling signals generated by the tracking unit a multiplier connected in parallel with said integrator; calculating means for providing signals corresponding to the estimated time-of-flight of a round to the multipliers, the calculating means being arranged to produce time of flight signals in response to a first input representing the slant range and second and third signals respectively representing correction values for varying muzzle velocities of rounds of ammunition of different kinds and correction values relating to variations in atmospheric conditions, means being provided for manually setting said values in the calculating means; and means for adding the aim-off correction signals produced by the respective multipliers to the respective controlling signals, at a time not preceding the commencement of the second phase of operation of the tracking unit.
 13. A gun according to claim 12, wherein the calculating means comprises an integrator, means for generating a first ammunition-dependent signal which is an inverse function of the muzzle velocity of the round and which is a function of the slant range, means for generating a second signal which is a function of the atmospheric condiTions, a multiplier for multiplying said first and second signals together, and an integrator whose input signals comprise said first signal and the result of multiplying said first and second signals and which integrator is provided with a negative feedback loop, the time-of-flight calculated signal consisting of the output of said integrator. 